The invention pertains to cryogenic freezing apparatus, and more particularly to such apparatus using a liquid cryogen, such as nitrogen, for cooling articles within a freezer apparatus. The present invention is particularly directed to a freezer apparatus which operates on a continuous stream of articles as opposed to such apparatus operating in a batch mode.
Various types of cryogenic cooling apparatus are known, and in general do a satisfactory job of cooling or freezing various articles. Two popular types of cryogenic apparatus in use today are commonly known as "spiral freezers" and "tunnel freezers". Both of these types of apparatus operate in a continuous mode, where articles such as foodstuffs or the like are continuously fed through the freezer apparatus with the stream of articles leaving the cooling apparatus being cooled or frozen, as desired. A cryogen immersion bath may be provided at the entrance end of the tunnel or spiral freezer, and if so, the freezer is commonly termed a "cryogen immersion freezer."
One particularly popular type of such freezer apparatus in use today employs a liquid cryogen, such as nitrogen, in which the articles are either fully or partly immersed, or alternatively the liquid cryogen is allowed to vaporize, thereby cooling the interior of the freezing chamber. Early designs of such freezer apparatus, known as "isothermal" freezers, operate at one low temperature throughout the cooling or freezing chamber. Commonly assigned U.S. Pat. No. 4,739,623 offered significant improvements to such freezers, greatly increasing the efficiency thereof by controlling the escape of cryogen vapors created in the freezer which, after being made to circulate through the interior of the freezing chamber, are allowed to escape through the warmest portions thereof. In the U.S. Pat. No. 4,739,623, a spiral conveyor is provided and a fan at the freezer entrance directs air toward the "stack" of coils of the spiral conveyor. Also employed in the freezing chamber are several circulating blowers to maintain desired cooling in various portions of the freezing chamber. One such blower is mounted adjacent the inlet to the freezing chamber and is controlled in response to temperature sensed at an exhaust duct. Ambient air surrounding the freezing chamber is channeled through the exhaust duct, and if the temperature thereof should rise in an undesirable manner, the blower adjacent the freezer inlet is decreased in speed to permit cold cryogen to escape through the entrance to the freezing chamber thereby blocking infiltration of the ambient air into the freezing chamber. Conversely, if the temperature in the exhaust duct should decrease more than a desirable amount, fan speed adjacent the freezer inlet is increased to blow more cold cryogen vapor toward the path through which the articles travel during freezing.
Commonly assigned U.S. Pat. Nos. 4,350,027 and 4,783,972 disclose tunnel-type cooling apparatus. Both apparatus use liquid cryogen cooling media and both have a series of blowers internal to the tunnel for facilitating cooling. U.S. Pat. No. 4,783,972 discloses a baffle and damper arrangement which is adjusted by a control motor in accordance with the temperature of vapor leaving an upwardly directed chimney or venting duct. If an elevated temperature is sensed, for example, the baffle and damper arrangement is adjusted to direct additional cryogen vapor toward the entrance end of the tunnel. The temperature sensed at the outlet of the duct is that of a mixture of cryogen vapor and ambient air which has infiltrated the entrance area. Thus, the temperature reading at the exit end of the duct is an indirect measurement of the condition at the entrance. While this arrangement has met with considerable commercial success, a more direct, i.e., more tightly coupled, control is desirable to further increase the efficiency of operation.
U.S. Pat. No. 4,276,753 provides another example of liquid cryogen freezing in a tunnel freezer. A conveyor belt is oriented in a generally straight line and passes through a tunnel enclosure within which liquid cryogen is dispersed by injection lines and circulated with a sequence of blower fans. A directional blower is located in the tunnel remote from the entrance end and circulates cryogen vapor in a counter direction, opposite that of the travel of the conveyor and the articles carried thereon. An exhaust blower is located at the top of a chimney or exhaust stack located above the entrance to the tunnel. A temperature sensor is mounted adjacent the downstream end of the tunnel and drives a control for the exhaust blower and the directional blower. When temperature adjacent the exit end of the tunnel increases above a preset level, the input of liquefied cryogen in the tunnel is increased and the operating speeds of the directional blower and exhaust blower are increased. Conversely, when the temperature sensed in the tunnel adjacent the exit end thereof falls below a lower preset, less liquefied cryogen is introduced into the tunnel and the operating speeds of the directional blower and exhaust blower are decreased. Thus, the exhaust blower located adjacent the entrance end of the tunnel is controlled by temperature sensed adjacent the exit end of the tunnel, fan speed being increased to prevent unacceptably high amounts of infiltration of ambient air into the freezing tunnel.
U.S. Pat. No. 4,403,479 discloses another example of a liquid cryogen freezing apparatus, comprising an upstream immersion bath for articles to be frozen, and a downstream freezing tunnel to complete the freezing or cooling process. An exhaust chimney or duct is provided at the outlet of the tunnel to draw cryogenic vapor from the immersion bath through the tunnel for further cooling of the articles, subsequent to the initial immersion. An amount of the liquid cryogen used in the immersion bath is allowed to spill, to create a cryogen vapor pressure, preventing infiltration of ambient air into the entrance of the cooling apparatus. The tunnel is operated without re-circulating fans and without vapor spray inlets. An exhaust fan adjacent the exit end of the tunnel and baffles within the tunnel are, however, employed.
Despite the advances discussed above, further improvements to liquid cryogen cooling apparatus are still being sought. For example, a significant improvement in efficiency of operation of a cooling apparatus can be obtained if vapor balance at the entrance to the freezing apparatus, where articles to be frozen are introduced, can be more closely controlled.
Other advances are being sought to provide a retrofit enhancement to existing mechanical cooling systems which are no longer adequate to handle increased product throughput. A liquid cryogen cooling tunnel could be employed at the entrance to the mechanical cooling system, but economies of operation dictate that the efficiency of the added cryogen system be sufficiently efficient to justify the added capital investment. Accordingly, liquid cryogen tunnel freezers of compact size and capable of economical efficient operation are being sought for such applications.